Is it a bird? Is it a plane? No, it's yet another superfood, fresh from the depths of a remote jungle and heading straight for our shopping trolleys. With their exotic names - acai, goji, maqui - and mysterious origins, they are storming the Western world. But what exactly does the term "superfood" mean? And do the berries make the grade?

"I think it might be like supermodels - there used to only be about five of them, and now everyone's a supermodel," says New Zealand Nutrition Foundation nutritionist Carolyn Cairncross. "And now everything seems to be a superfood."

Google "superfoods" and there are more than nine million results. The term has been applied to almost any food product in the desire to convince consumers that fruits, vegetables and even chocolate are super choices for their daily diet.

There is, in fact, a serious divide in the definition of the term, although a general understanding is that it refers to foods that are rich in nutrients.

Dietitian Sharon Carey argues that "all natural foods are super - what's important is to enjoy a wide variety of foods so you get a balance of all nutrients".

But Matakana Superfoods owner Kevin Glucina, says the term should be applied only to a certain group of very special foods.

"The term is being used loosely at the moment ... You need to have scientific analysis to show superfoods offer more than others."

Perhaps the most important tool in defining a superfood is by measuring their levels of antioxidants, done via the Oxygen Radical Absorbance Capacity (ORAC) scale. "The ORAC value is the 'cut and polish' power to keep your cells clean," says Glucina.

According to him, the original superfood was the apricot with an ORAC score of 1100 u mol/100g, while the current "king of superfoods" - the maqui berry - has 95,000u mol/100g. "Which means that the maqui berry has 100 times more powerful antioxidants than an apricot for cleaning up oxidative substances in your cells that are responsible for so many health problems."

So what do these wonder foods do? The claims for products such as the goji berry, acai berry, and chia seed get particularly outrageous when it comes to some online retailers. From being able to support a quicker metabolism, protect eyesight and improve fertility to reducing blood pressure, fighting arthritis and regenerating cells, these superfoods can start to sound like miracle cures. It's this danger that Consulting Dietitians' founding director Sarah Ley worries about.

"There's no quick fix, and I think a lot of people would like to take these foods for that reason," she explains. "From a public health perspective, you get people clinging on to the idea of this magic bullet for health problems, and that's not good at all."

In 2007 the EU banned the use of the term "superfood" on products unless it was accompanied by a specific authorised health claim.

In New Zealand, there is no such ban, although there are strict rules surrounding the misrepresentation of a product or its health benefits which are policed by the Commerce Commission and the national medicines safety department, Medsafe.

Three years ago, Medsafe investigated one company over inaccurate claims it made about its goji juice, while New South Wales' authorities also looked into some Australian and New Zealand companies' assertions that "superjuices" could help treat diseases such as cancer and diabetes.

Glucina believes his products do support health and there are scientific studies to back up those claims, but he acknowledges sellers of superfoods are "in a grey area" when it comes to advertising their benefits.

"We can't claim they are medicines, but we do know the nutrients in fruit and vegetables are very beneficial to health ."

Cairncross says although there may be research, the methods don't always stand up to rigorous investigation.

As Ley points out, health and food experts "simply don't have enough information yet to say these things are good for you".

Should we be eating them, then? The short answer is yes.

Eating foods rich in nutrients is essential for good health, but making sure your diet is balanced is still the most important rule.

Glucina says he believes in "eating fresh, seasonal - organic where possible - foods and then in addition you can have a cross-section of superfoods".

The food and diet experts agree, although Carey believes you needn't shell out a fortune on superfoods to get similar benefits. "A gingko berry may be high in antioxidants but so are kiwifruit. In fact, having a range of colours of fruit and vegetables will ensure you get a balance of antioxidants."

And if you are turning to superfoods to aid weight loss or burn fat, don't be tempted by the promise of a magic slimming super-smoothie.

"If you have less butter or margarine on your toast in the morning, or do an extra 10 minutes at the gym, you will lose fat," Cairncross says. "But that message we're giving is bland and a bit boring, theirs sounds much better."

Superfoods to rule them all

* Acai berry
Sourced from the Amazonian rainforest, it has an ORAC value of about 60,000u mol/100g, and numerous other nutrients. Available in a powder extract.

* Maqui berry
Originally from Patagonia, it has the highest known ORAC score and is available in a powder extract.

* Chia seed
A super-seed from Central America that boasts high levels of proteins, amino acids, omegas, vitamins and minerals.

* Goji berry
An ancient medicinal food in China and Tibet, it has an ORAC value of 25,000u mol/100g.